The use of portable electronic devices has increased dramatically in recent years. For example, portable electronic devices such as notebook computers, cellular telephones, and Walkman.TM. type radios and tape and compact disk players are in common use. In addition, portable electronic devices such as data phones and personal digital assistants (PDAs) which combine communications and computing capabilities are being developed and sold. The use of small, light weight rechargeable batteries to power these portable electronic devices is also increasing. Accordingly, there exists a need for low-cost, small, light weight battery rechargers to recharge these batteries quickly and efficiently.
In the past, single rate battery chargers have been used. For example, a current limited DC source can be shorted to the battery pack. A single rate charger, however, may be unable to provide 100% charging. In particular, many batteries may need a fast-rate charge followed by a low-rate charge to reach a fully charged state. For example, a fast-rate charge may be followed by a C/10 (one tenth the current of the fast-rate charge) time-limited charge to insure 100% charging of a battery under all conditions. This fast- and low-rate charging may be particularly desirable when charging medium and large sized battery packs such as those used to power notebook computers, personal digital assistants, and data phones. The use of fast- and low-rate charging may also be desired when simultaneously charging the battery and operating the portable electronic device. In addition, variable rate charging may be beneficial when used with batteries having particular chemistries such as nickel based and lithium based batteries.
Accordingly, switch-mode battery chargers have been used to provide fast- and low-rate charging. In a switch-mode battery charger, a switch and a filter are arranged in series between a DC source and the battery to be charged. The rate of charge is determined by a duty-cycle at which the switch is turned on and off. A higher duty-cycle provides a faster-rate charge, and a lower duty-cycle provides a lower-rate charge. To reduce the AC ripple of the charging signal provided to the battery, the filter typically includes an inductor in series between the switch and the battery, and a free-wheeling diode between the switch output and ground.
To allow switching over the full range of charging rates, a relatively large inductor is generally used. In particular, a conventional battery charger uses an inductor which does not saturate over the full range of charging rates. The use of a large inductor may increase the size, weight, and cost of the charger. In addition, energy stored in the inductor may result in undesirable transients when switched at higher charging rates. The switching transistor and free-wheeling diode should also be sized to accommodate the high peak current due to the energy stored in the inductor during fast-rate charging.
Another approach has been to use dissipative elements to vary the charge rate. For example, a passive component such as a resistor can be used to control the current to the battery. Alternately, an active component, such as a transistor operating in a linear mode, can be used to control the charging current to the battery. When using medium and large sized battery packs, however, the power dissipated in the dissipative element may be excessive. Accordingly, a dissipative charger may draw excessive power, generate excessive heat, and have reduced reliability. To compensate for the heat and power, a relatively large dissipative element may be required.
According to yet another approach, a fast-rate charge can be provided by shorting a current limited DC supply to the battery, and then a low rate charge can be provided by using a dissipative element. Such an approach suffers the shortcomings discussed above with regard to the dissipative element as well as the increased complexity of combining the two approaches.
Notwithstanding the battery chargers discussed above, there continues to exist a need in the art for improved battery chargers. In particular, there exists a need in the art for small, low-cost, and light weight battery chargers which can be included in a portable electronic device.